Materials Matters

Vol. 3 No. 1

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President's Journal

by Leslie Evelyn

It's here again: the quarterly, biannual, annual, periodic, sporadic issue of Materials Matters. We somehow managed to coax our editor to compile this issue after much grumbling and apathy. My apologies to our few (about five) but faithful readers; if it were entirely in my hands, I would promise a timely and regular publication. We have tried to explain in a somewhat non-technical manner (that's hard for us) how we dabble in fabricating heart valves and how we try to guide light through not-so-exotic materials, as well as informing you what has been taking place in and around our MRS chapter. Since this newsletter seems to be the only window on the ongoing materials research at Alabama A&M University, we highly recommend that you read and enjoy this issue. Check out our incomplete homepage at cim.aamu.edu.

1995 Summer visiting students

by EkW

The Irradiation Center was full during the summer of 1995 with three students visiting from Lyon, France and one from Stanford University. Guillaume Aymard, Isabelle Ligier and Virginie Normand followed in the footsteps of Cedric Ray, Marc Martorana and Arnaud Crastes who spent previous summers at AAMU. The three Frenchies were provided a form of transportation that worked for most of the summer: Eric Williams' 1971 Buick Estate Wagon with special fresh air ports around the wheels and a leaking fuel line. They made it back to France safely but the car is dead. Kamyar Madani spent his summer off from Stanford setting up the Irradiation Center's Web pages. If it had been up to us we would still be trying to figure out how to turn the computer on.

AAMU-MRS' homepage

by EkW

The AAMU-MRS Chapter has a home page (http://cim.amu.edu:80../mrs/mrs.html or just search for CIM, click on www and follow the link) at the Irradiation Center site with some information about us and links to the national MRS and to the Irradiation Center. Photos of most the students, faculty and staff of the AAMU-MRS and of the Irradiation Center are on our homepage. The site also includes descriptions, some illustrated, of current and past research projects and information about our collaborators from Brazil, France and Germany. Kamyar Madani spent most of last summer setting up the site.

Fall MRS

by EkW

The contingent of six persons from AAMU went to Boston Mass for the Fall 1995 MRS meeting. Five of them drove twenty hours each way to get there and back. Actually, two of them drove and the other three rode. We presented a mess o'papers and used our symposium aide money to eat good food. What wasn't covered by the paid work as symposium aides was made up for by some per diem money that we managed to get.

IBA

by EkW

Leslie Evelyn, Eric K Williams, D. Ila and Robert L. Zimmerman presented papers at the 12th International Conference on Ion Beam Analysis held in Tempe, Arizona from 19 to 23 May, 1995. In order to get cheap airfares Leslie and Eric flew to Phoenix two days prior to the conference and took advantage of the extra time to look out over the Grand Canyon and peer through the telescope at Lowell Observatory in Flagstaff. They recommend La Bella Via for breakfast or lunch. If you order the Swedish oat pancakes order just one unless you are very hungry. They also serve the best hot cocoa that Eric has found to date in the United States. After completing the main portion of their trip Leslie and Eric returned to Phoenix for the IBA conference. The papers that they presented will be published in an upcoming issue of Nuclear Instruments and Methods in Physics Research B. Eric's paper was on RBS and nuclear reaction analysis of He bombarded lithium niobate and Leslie's paper was about 4 pages long. Many other people attended the conference. We met some of them and ignored the rest. Leslie and Eric also went to the zoo.

Renovations, France(?), MPI

by EkW

The Howard J. Foster Center for Irradiation of Materials is expanding and growing and threatening to take over the campus. The CIM lab and the surrounding buildings are to be gutted and rebuilt as one nice shiny new laboratory facility. Work has begun already. The accelerator lab denizens will become homeless sometime in May and float about the campus for a few months until renovations are complete. The improvements are funded by NSF and state grants from special funds for such projects. The homeless EkW and Leslie Evelyn will try to convince Université Claude Bernard to take them in for a month or two over the summer; after all, we've had five(count 'em, five) of their students here. Professor D. Ila is escaping to Germany's Max Plank Institute for the summer

IBMM

by EkW

Ion Beam Modification of Materials '95. Canberra, AUSTRALIA. A very long plane flight. Four CIMers attended: EkW, D. Ila, RLZ, and Dan Nisen, our technician. Eric went early and stayed late. He walked all around Sydney until the others arrived and then they drove on the wrong side of the road to Canberra, getting lost only a couple of times with only a little bit of yelling. The conference was organized without parallel sessions so one did not have to choose between equally attractive sessions. That the conference was held in February was terrific for attendees from cold northern climes. Quite a few scientists took two or three weeks to attend a week-long conference. Several workshops were held after the conference: Eric attended one on Optical Materials in Bateman's Bay where he and the other workshoppers were forced to eat sumptuous meals for three days. The sole Canadian didn't want to go home, noting that the temperature was +27C in Bateman's Bay but -27C in Toronto. Robert Zimmerman, Dan Nisen and EkW went to Melbourne to attend a workshop on Focused Ion Beams and to meet with George Legge from whose organization the CIM has purchased a focused ion beam system that will be installed in the fall of this year, after renovations to the Irradiation Center and adjacent buildings are completed.

Selected Recent Work and Publications: GaAs and Carbon

Channel Waveguides by MeV Ion Beam Mixing

by Thomas Taylor

Efforts at the Howard J. Foster Center for Irradiation of Materials (CIM) at Alabama A&M University have been focused on creating localized modifications in optical as well as electro-optical material systems using ion beam processing. Joint collaboration established with Paul R. Ashley at the U. S. Army Missile Command Center and David B. Poker at Oak Ridge National Laboratory have been instrumental in achieving our current level of success. One project that is currently under investigation deals with fabricating channel waveguides in the GaAs/AlGaAs material system using MeV ion beam bombardment. There is increasing interest in utilizing III-V compound semiconductor waveguides as either active or passive components in optoelectronic devices. The interest has been motivated by the potential for monolithic integration of optoelectronic components. In a waveguide, a focused laser beam is confined in one (planar) or two (channel) dimensions to values of the order of the wavelength of laser light used for distances determined primarily by the propagation losses. Low loss electro-optical waveguides have potential use as interferometric modulators. Channel waveguides in particular are attractive for integration because they can be fabricated from a variety of materials and can readily be tailored to meet a specific task requirement by a number of techniques.

Ion beam processing is a widely used technique employed to create localized material alterations in the band gap and refractive index of GaAs/AlGaAs quantum well waveguides. A typical single quantum well waveguide structure consists of a high index guiding layer containing undoped GaAs (core) bounded on the top and bottom by layers of doped AlxGa1-xAs which form the low index claddings, or vertical confining layers. This type of planar waveguide geometry has limited applications because of its one dimensional confinement capability. Changes in the refractive index induced by high energy ion beams can provide lateral waveguide confinement of laser light through the guiding region in addition to the vertical confinement provided by the cladding layers. Ion beam processing offers the advantage of producing precisely defined modifications in specified regions around a selective mask while leaving other regions undisturbed. Fabrication of channel waveguide geometries in the GaAs/AlGaAs system have expanded application usage in optoelectronic integrated circuits (OEICs).

Energetic ions undergo two major mechanisms of energy loss called nuclear stopping and electronic stopping as they travel within materials. Nuclear stopping is characterized by screened Coulomb collisions occurring between the incident ions and target atoms whereas electronic stopping results from interactions between the incident ions and target electrons. The relative contribution of both stopping powers in a specific material depends primarily upon the incident ion energy and mass in combination with the atomic mass of the target. In a bi-layered material a large number of nuclear collisions occurring at the interface can cause disordered regions and mixing of the two layers. These disordered regions exhibit modified material properties.

The exact mechanism by which the electronic stopping power alters material properties is not fully understood at this time. Attempts to formulate a quantitative model have proved to be a complicated and difficult task.

The majority of work being conducted on layer intermixing concentrates on using ion beam energies in the range of 5 to 800 keV to modify the optical properties GaAs/AlGaAs structures. Low energy modification schemes of this nature require a high temperature post-annealing procedure (~850 oC for GaAs/AlGaAs) to enhance the compositional intermixing due to the shallow penetration depth of the incident ions. The high temperature annealing step can restrict future device fabrication processing and the shallow deposited ions can promote undesirable doping effects within the layers. Efforts to minimize these factors drive our investigation into the use of high energy ion beam bombardment to alter the optical properties of planar GaAs/AlGaAs waveguides. We have successfully demonstrated that the electronic stopping power of 10 MeV oxygen ions can change the refractive index in regions of planar GaAs/AlGaAs waveguides to create channel waveguides. Recent manuscripts detailing our preliminary successes include:

T. Taylor, D. Ila, R. L. Zimmerman, P. R. Ashley and D. B. Poker, Mater. Res. Soc. Symp. Proc. 373 (1995).

T. Taylor, D. Ila, R. L. Zimmerman, P. R. Ashley and D. B. Poker, Mater. Res. Soc. Symp. Proc. 396 (In Press ).

We are optimizing the ion beam parameters necessary to induce optical changes. We are also employing various analytical techniques to investigate structural modifications within the bombarded waveguide samples.

PRODUCTION OF HEART VALVES FROM GLASSY POLYMERIC CARBON

by G. M. Jenkins, D. Ila, and H. Maleki

ABSTRACT

When certain resins are pyrolized they transform into a glassy polymeric carbon (GPC) with no change in shape. Using this process all-carbon heart valves have been made by rapid molding of the component pieces out of precursor resin, assemblage in the resin stage and pyrolysis to at least 1000o C to form an accurately articulated device. A heart valve with two occluders set in a carbon ring manufactured out of highly polished carbon has been tested with an excellent record of wear, fatigue, resistance and biocompatibility, especially in contact with blood. The material is brittle and susceptible to stress concentrations. We are seeking to increase the strength by varying the resin shaping process, by further heat treatment up to 3000oC and local strengthening using bombardment in-house with carbon and gold ions.

INTRODUCTION

The production of polymeric carbon (PC), typically derived from a phenolic resin precursor, is important for various applications including biocompatible prostheses such as heart valves, and percutaneous and drug delivery devices. For these applications, it is important to have controlled pyrolysis. For others, it is important to remove all impurities to leave a lightweight, fully carbonized product which can be used for any device operating inside a living species [1-6].

Precursor resol C7H8O2, converts to fully cured phenolic resin C7H6O of specific gravity 1.25 on heating at 150C to 200C. This resin further transforms without disruption and with no change in shape into a porous but impermeable glass-like polymeric form of carbon ("glassy carbon"), of specific gravity 1.45 on heating to 1000C [1,2]. Heat treatment to 550C produces an intermediate conducting hydrocarbon which improves progressively in conductivity as the temperature increases and hydrogen is released. The final product is long ribbon-like graphene molecules of sp2 carbon atoms aggregated locally to form sub-crystalline domains arranged randomly in space [1].

Figure 1, a through c, shows how polymeric carbon can be made from a resol, which is a compound of phenol and methanal. Curing at 160oC produces a cross-linked thermoset phenolic resin. Heat treatment of this resin to 500C causes these units to combine to form an intermediate ladder structure. Further heat treatment at higher temperatures will cause dehydrogenation and result in an interconnected aromatic network of graphene ribbons.

It has already been shown [7,8] that articulated devices suitable for the replacement of heart valves can be made from glassy polymeric carbon. Precursor parts have been molded under high pressure and then carbonized. The mold reproduction is so good that no further machining or polishing is required to produce a functioning articulated device. The molding operation and the firing process have been made automatic and so the overall fabrication cost is low. Long term mechanical testing of the device has shown that wear resistance is high and that fatigue effects are negligible. The main problem is the brittleness of the material; thus the strength is sensitive to surface flaws. This effect is particularly noticeable at stress concentrations which are unavoidable in the chosen design. We plan to remedy this problem by ion bombardment of areas that are subjected to high local stress during insertion and during major convulsive muscular trauma.

In contrast, when polymers are used in a device with moving parts, cracking and exfoliation of the surfaces may occur. This is particularly true for highly crosslinked polymers; their surfaces are brittle and may crack due to friction wear. The beginning of a crack is strongly determined by the compliance of the contact pair which affects the deformation of the bulk polymer underneath the harder surface layer. It has been shown that ion bombardment of polymers changes both their microstructure and composition [9-11] and that these structural modifications may increase the abrasive wear resistance of ion beam modified GPC [12,15].

Glassy polymeric carbon is both biocompatible and inexpensive. Depending on the pyrolysis, GPC can be made into a hard impermeable solid with a highly polished surface for cardiovascular devices or sponge-like on a nanopore scale to host several drugs which can be incorporated into the pores and released in a controlled manner [5,6]. Our objective is to find means to increase local strength and toughness by ion implantation of carbon, for instance, which will increase the density above 1.45 Mg/m3 [16]. We have laid down well bonded surface layers of malleable gold to reduce stress concentrations at any surface fault. We have tried to increase the overall strength by suitable heat treatment to higher temperatures up to 3000oC and by varying the shaping operation of the resin precursor.

EXPERIMENTAL PROCEDURES

Polymeric carbon samples are prepared by three methods: molding, spraying and spin coating resin, thinned with an enol, into a predetermined shape on heating. The cured resin is then removed from the holding substrate. After cleaning in strong acid and distilled water the shaped device can be polished if necessary. These devices are prepared at pre-determined pyrolysis temperatures depending on their applications. Using graphite resistance furnaces we are able to heat treat specimens up to 3000oC. The last traces of hydrogen are removed and also trace impurities. Heat treatment to these high temperatures does not change the structure much but X-ray diffraction peaks become sharper and the images in electron transmission are better resolved. Electrical resistivity is reduced slightly and the band gap approaches zero.

The heart valves shown in Figures 2 and 3 are made of GPC. The first is derived from polyfurfuryl alcohol and the second from phenolic resin. The first has one moving component (occluder) and the other has two, with both components held in a fixed body. The moving and fixed parts are molded, fitted together and then heated to 1100oC. The final products are articulated devices in which the flaps move with minimum friction while preserving occlusion at the appropriate part of the cycle. The most sensitive regions are the hinges, which in the second valve consist of pairs of pivots and pivot holes in each side of the valve. The hardness and strength of these regions may be improved by increasing the material density above its normal value both by careful heat treatment above 1000oC and by local bombardment with carbon ions. Surface toughness can be improved by local layering with malleable gold.

We have not been able to increase strength appreciably by varying the shaping process or increasing the temperature during pyrolysis. Our results on carbon bombardment are inconclusive but the latest data show that hardness and wear resistance are greatly improved. We have initiated the bombardment of glassy carbon with gold ions and shown that gold penetrates deeply into the carbon allowing us to use electrochemical techniques to lay down a superficial layer of gold which will be strongly bonded to the carbon [17]. Without the initial ion bombardment the gold layer would be weakly bonded to the carbon because chemical bonding between gold and carbon is impossible.

CONCLUSIONS and RECOMMENDATIONS

It has been demonstrated that it is possible to fabricate complicated articulated mechanisms suitable for heart valve replacement out of glassy carbon derived from phenolic resin. The process and materials are not expensive. It has been shown that wear and fatigue resistance are excellent but that the low strength and brittleness of such devices limit their present application. In order to improve this we have looked into the possibility of using alternative fabrication techniques for the resin precursor and have tried different pyrolysis temperatures and conditions. These have not been successful. We are now investigating the possibility of local strengthening by bombardment with carbon which produces local densification. We have also initiated a process of gold bombardment to bury the gold deep into the glassy carbon surface, allowing us to use electrochemical techniques to produce a superficial layer of gold which, being malleable, should lower the sensitivity of the surface to stress concentrations.

ACKNOWLEDGMENTS

This project is supported by National Science Foundation EPSCoR-II (Alabama) Grant No. EHR-9108761 and the Howard J. Foster Center for Irradiation of Materials at Alabama A&M University.

REFERENCES

G. M. Jenkins and K. Kawamura, Polymeric Carbon - Carbon Fiber, Glass and Char, (Cambridge University Press, 1976).

D. Ila, G. M. Jenkins, L, R. Holland, A. L. Evelyn and H. Jena, VACUUM, 45, No. 4, 451 (1994).

D. Ila, A. L. Evelyn, H. Jena and G. M. Jenkins, Carbon 32, No.7, 1211 (1993).

D. Ila, G. M. Jenkins, R. L. Zimmerman and A. L. Evelyn, Mat. Res. Soc. Symp. Proc. 331, 281 (1993)

H. Maleki, G. M. Jenkins, D. Ila, and R. L. Zimmerman, Mat. Res. Soc. Symp. Proc. 371, (1994).

R. L. Zimmerman, D. Ila, G. M. Jenkins, H. Maleki, and D. B. Poker, Nucl. Instr. & Methods B, (1995).

G. M. Jenkins, "Biomedical Applications of Carbon and Graphite", Clin. Phys. Physiol. Meas., 1, 171 (1980).

G. D. Angelini, C. Price & G. M. Jenkins, Surgery for Heart Valve Disease, Bodnan, pp517-522 (1989).

D. Ila, A. L. Evelyn and G. M. Jenkins, , Nucl. Instr. & Methods, B91, 580 (1994).

D. Ila, A. L. Evelyn, and G. M. Jenkins, Mat. Res. Society Symp. Proc. 321, 441 (1993)

D. McCulloch, A. Hoffman, and S. Prawer, Phys. Rev. B50, 5905 (1994)

K. Yoshida, K. Takahashi, K. Okuno, G. Katagiri, M. Iwaki, and A. Ishitani, App. Phys. Lett., 52, 1046 (1988).

M. J. Kenny, J. T. A. Pollock and L. S. Wielunski, Nucl. Instr. & Methods. B39, 704 (1989).

E. H. Lee, Y. Lee, W. C. Oliver, and L. K. Mansur, J. Mater. Res. 8, No. 2, 377 (1993).

E. H. Lee, G. R. Rao, M. B. Lewis, and L. K. Mansur, J. Mater. Res. 9, No. 4, 1043 (1994).

D. G. McCulloch and S. Prawer, J. Appl. Phys., (1994, submitted for publication).

R. L. Zimmerman, D. Ila, G. M. Jenkins, H. Maleki and D. B. Poker, Nucl. Instr. & Methods B, (1995, submitted for publication)

STUDENT NIGHT

by Thomas Taylor

In February the Huntsville Chapters of ASM International and SMPTE sponsored a student night. Students from Huntsville City and Madison County High schools and Alabama Universities attended as well as local area business professionals. The meeting's goals were to promote science and engineering interests in high school students and materials-related study among college students. The attendees enjoyed pizza and soft drinks as they browsed high school, college and business displays. A number of attendees stopped by the AAMU- MRS Chapter display to gather membership information. Dr. Frank Six delivered a lively presentation on "The Hubble Space Telescope, an Incredible Discovery Machine." Lastly, the sponsors awarded door prizes prior to final career and study discussions among the attendees.